1. Field of the Invention
This invention is directed to methods and composite materials for purifying fluid streams. For example, fields of the invention include flue gases emitted from combustion sources (e.g., coal-fired power plants, waste-to-energy facilities, medicinal and similar incinerators, and industrial manufactures) and for purifying ground, surface, and/or industrially processed waters. More particularly, the invention relates to removal of mercury and other contaminants from a fluid stream by adsorption and either subsequent or continuous catalytic and photocatalytic oxidation using catalyst and photocatalyst impregnated or doped sorbents (e.g., silica-gels).
2. Description of the Related Prior Art
Mercury emission from combustion sources is a significant environmental concern. When mercury is released into the atmosphere, it can be transported by wind and then through direct deposition can accumulate in surface waters. In the water, biological processes can transform mercury (typically elemental mercury) into methylmercury, which is highly toxic and can bioaccumulate in fish. Therefore, preventing the release of mercury into the environment is very important.
The largest emitters of mercury are coal-fired electric utility plants, which account for an estimated more than 90% of all anthropogenic mercury emissions. Mercury is also listed as one of the 189 hazardous air pollutants (HAPs) in the 1990 Clean Air Act Amendments (CAAA). Regulations are being set for future emission standards for combustion sources to be implemented as early as 2007.
The unique feature of mercury emission that differs from other toxic metals results from mercury's 5d106s2 closed shell electronic structure that is isoelectronic to He (1 s2), and is accordingly highly stable in its elemental state. As a result, unlike other toxic metals, the dominant form of mercury in combustion exhaust is elemental mercury (Hg0) vapor, unless chlorine is present. Since Hg0 is insoluble, gas removal devices, such as scrubbers, are also ineffective for its removal. Similarly, particulate removal devices (e.g., baghouses and electrostatic precipitators) are also highly ineffective.
In recent years, numerous studies for enhanced mercury removal from combustion sources have been undertaken. Generally the methods used in these studies were based on either adsorption or oxidation. Currently, the maximum available control technology (MACT) for mercury is powdered activated carbon injection. However, its predicted use is limited because of questionable costs, presently low capacity, low applicable temperature range, and problems associated with collection and regeneration of the carbon. This approach has been estimated to cost about $2-5 billion annually to implement in U.S. coal-fired power plants.
In addition to activated carbon, calcium-based sorbents such as hydrated lime have also been considered. However, calcium-based sorbents provide poor efficiency for mercury removal unless they are modified with fly ash. Studies have also been carried out using zeolite and bentonite, but they have demonstrated very low capacity for mercury.
The use of an advanced oxidation process has also been investigated for mercury removal. Heterogeneous photocatalysis is one such method that utilizes a semiconductor in the oxidation and mineralization of pollutants, either in the air phase or water phase. When the surface of the photocatalyst absorbs a specific amount of energy (usually a photon), an electron from the valence band is promoted to the conduction band, thereby leaving a positive charged “hole” in the valence band. Reactions with these electron-hole pairs result in the formation of hydroxyl radicals (OH−), which are very reactive oxidizing species. Titanium dioxide (TiO2) is one such semiconductor/catalyst that can be activated when irradiated by UV light.
Wu et al. (Env. Eng. Sci. 1998;15(2):137-148) and Lee et al. (AlChE J 2001;47:954-961) used TiO2 nano-aerosols generated in-situ in combustion systems to effectively transform elemental mercury into mercuric oxide. This process was reported to have high efficiency, but a major limitation related to separation and regeneration of the mercuric oxide-loaded TiO2 aerosols.
Therefore, an economical solution that can more efficiently capture mercury compared to the technologies discussed above offers the potential to significantly decrease the estimated costs to meet pending regulations. In addition, the solution should be engineered such that it could be easily implemented in existing coal-fired power plants and similar installations. Not to be bound by theory, but a viable location for the technology derived herein would be to insert the technology between the electrostatic precipitator or baghouse and the effluent stack for coal-fired power plants, although the technology is not limited to this location. In addition, it is conceivable to micronize (create a fine powder with diameters less than 45 μm) and inject the material in to a flue duct.